High-brightness edge-emitting semiconductor lasers are lasers that are characterized by high power and high beam quality of the output radiation. These semiconductor optical sources are attractive for use in a variety of applications including, for example, industrial material processing, pumping of fiber amplifiers, fiber and solid state lasers, free space communications, second harmonic generation, medicine, laser printing, lidar. The emission of high-brightness lasers shows conveniently an approximately constant spot size over significant propagation distances (far-field zone) and is suitable for a variety of direct applications, without using external focusing optical systems of large complexity.
High-brightness lasers are supposed to radiate high power and simultaneously have small spatial angle (small divergence) of the radiation in both vertical direction (direction of the epitaxial growth) and lateral direction (parallel to the epitaxial plane). The brightness of a laser source is generally understood as the power divided by the mode area in the focus and the spatial solid angle in the far-field. As compared to diffraction-limited beams, for non-diffraction-limited beams the brightness is reduced by the product of the M2 factors of the beam quality for the vertical and lateral direction. The M2 factor is defined as the product of beam radius at the beam waist and the far-field beam divergence angle divided by the corresponding product for a diffraction-limited Gaussian beam with the same wavelength.
Typical high-power edge-emitting lasers have a large divergence in the vertical direction and a low beam quality in the lateral direction. They consist typically of a vertical sub-um-sized single-mode epitaxial layer waveguide of III-IV or II-VI semiconductor materials and a multi-mode laterally etched waveguide of a width of several tens of um. This results in the M2 beam quality factor in the lateral direction which is typically much higher than in the vertical direction due to the emission of a multitude of higher order modes. Such a radiation cannot be focused to a small spot, needed for most applications, even using complicated external focusing optical systems.
A number of different approaches are known to obtain high-power semiconductor lasers with small divergence in the vertical direction. A concept of large optical cavity with low index-contrast GaAs/AlGaAs waveguide in the vertical direction was developed which resulted in ˜1 W single-mode power and a vertical beam divergence of ˜10° (see Ref. [1] and references therein). Another concept of high-brightness semiconductor lasers employs a thick vertical waveguide formed by a quasi-periodic multi-layered GaAs/AlGaAs sequence known as a vertical photonic bandgap crystal (PBC lasers) [2]. This approach has allowed generating a narrow vertical beam divergence down to 5° or less at different wavelength ranges from the visible to the near-infrared. A single mode cw output power of 2.1 W at 980 nm, and as a result, highest brightness to date for conventional semiconductor lasers of 0.35 GW/cm2sr was reported. The PBC laser approach is applicable also to other laser diode material systems e.g. based on (Al,Ga,In)N/GaN semiconductor materials, generating light in the UV-green range. An alternative concept is based on a leaky-wave laser design providing output of the laser emission through a thick transparent substrate with extremely narrow beam divergence of <1° [3-4]. However, development of this concept has been restrained by the existence, besides of such a low divergence substrate beam, also of large divergence beam coming from a conventional narrow vertical waveguide including the gain medium. The latter typically contains a considerable fraction of the output power and no high output power of the leaky-wave lasers in the substrate mode was reported yet.
The output power of edge-emitting semiconductor lasers and output beam quality in the lateral direction are defined by the lateral laser structure which provides the optical field confinement in this direction and simultaneously supplies the pump current to the active area. Typically the lateral laser structure is comprised of an etched stripe characterized by different refractive indices in the stripe cross-section and coinciding with the current contact. With increasing stripe area, the pump current and hence the emitted laser power increase, as the power is approximately proportional to the current. The latter relation is valid up to the onset of thermally induced current-spreading and gain hole-burning nonlinear effects. Narrow single stripes provide a single-mode radiation with single-lobed far-field in the lateral direction but yield no high power, since no high enough currents are possible. Widening of the stripe allows to launch high current and increase the output power, however inevitably worsens the output beam quality due to lasing of higher-order lateral modes of a wide waveguide stripe characterized by multi-lobe far fields.
Several approaches are known to find a compromise between the power and the beam quality in the lateral direction for edge-emitting semiconductor lasers. A tapered stripe geometry presents an effective design to combine the desirable operational characteristics of high power and narrow output beam with simple, low cost device fabrication (see Ref. [5] and references therein). For tapered stripe waveguides, it is considered that the fundamental lateral mode corresponding to the narrow end of the taper have diffraction-free propagation and amplification along the whole extent of the stripe, including the wide end of the stripe. Higher-order lateral modes of the wide taper part are discriminated at the narrow end and make up only a small portion of the laser emission at the output facet. The output emission of the tapered laser consists mostly of the fundamental lateral mode which corresponds to the wide taper part near the output facet, showing a single-lobe far field. The output power can be rather large due to the large total contact area. Both these factors made tapered lasers to be a promising approach for development of high-brightness diode lasers. A modification of the tapered stripe lasers is also known, where the wide stripe of the diode laser has a narrow part in the middle leading to the shape of a “bow-tie” waveguide [6].
Similar mechanisms of fundamental lateral mode selection and higher-order modes discrimination are utilized in double bend stripe waveguides [7]. For relatively wide stripes at the rear and output facet, the fundamental lateral mode provides a much smaller loss at the stripe bends than the higher-order lateral modes. As a result, the output emission has a single-lobe far field in the lateral direction with potentially higher power due to the stripe, which could be made wider as compared to straight stripes providing a comparable output beam quality.
Attempts to increase the brightness of edge-emitting lasers were also made by using multi-stripe field-coupled laser arrays with a total power being nearly proportional to the single-stripe output power times the number of stripes [8-13]. The lateral far-field divergence of multi-stripes being defined by the total width of the device is narrower than that of a single stripe, leading to another way, which allows increasing the brightness. However, in praxis the field-coupled multiple stripes typically radiate at many lateral modes with multi-lobe far-field patterns, the number of modes being equal to the number of stripes, provided each stripe is laterally single-mode. These multiple modes (so called supermodes) have different phase relations between the electromagnetic fields emitted from each of the stripes considered as individual light sources. Only the mode with equal phase from all stripes (in-phase mode) or the mode with pi-shifted phases (out-of-phase mode) along the whole array cross-section are of interest for achieving a high brightness. The in-phase and out-of-phase supermodes have single- or double-lobe far fields, respectively, and are suitable for applications. A presence of other supermodes with different phase shifts deteriorates the output beam quality, since they show a multi-lobe far field. For coupled laser arrays, thus the challenge remains to achieve a high lateral beam quality, since the multiple supermodes of the arrays equally contribute to the output, because they are degenerated by confinement factors and losses.
Different types of field coupling between the stripes were considered in order to eliminate this degeneracy—evanescent-wave coupling, leaky-wave coupling, diffraction coupling, e.g. by means of external mirrors (see Ref [12] and references therein). It was done by different modifications of the stripe processing, resulting in different ratios of the refractive indices within stripes and inter-stripe areas. Another realization for the same purpose is known using arrays with coupling the stripes by means of Y-shaped junctions or combination of X- and Y-shaped junctions [12]. Non-uniform stripe arrays in the lateral direction were also proposed [13] to eliminate the lateral mode degeneration. In contrast to these realizations of stripe arrays with the near-order coupling between neighboring stripes, Talbot-type spatial filters were employed with far-order field coupling of the stripes [14].
However, for all these laser diode arrays the discrimination of multiple lateral modes in favor of the desired single mode remains weak and/or too sensitive to the variations of the laser operational conditions (as in the case of diffraction coupling and Talbot filter). Differences obtained in the confinement factors and losses of the in-phase or out-of-phase mode (as well as any preferable single lateral mode) as compared to other lateral modes are too small.